We've suspected it for awhile now, and now it's confirmed that the bacterium Yersinia pestis was responsible for the devastating plague that wiped out a third of Europe 650 years ago. But this ultimate killer started as something far different.

The deadly plague was once Yersinia pseudotuberculosis, a minor bacterium that doesn't do much more than cause some mild discomfort in the stomach. Both of these bacteria are still around today - Y. pestis still gives about 1,000 to 3,000 people the Black Death every year, and it's still capable of killing within three to five days. The only thing that has changed is its ability to spread. Thankfully, we haven't seen anything even remotely like the outbreak that killed a third of all Europeans in just five years between 1348 and 1353.

Y. pseudotuberculosis has also hung around, but you wouldn't know it. I mean that quite literally - most people who have it don't even show any symptoms of the disease. And yet DNA sequencing of these two bacteria reveal that they're almost identical. How can two such similar bacteria produce such radically different results? Northwestern University researchers say the answer isn't in the DNA at all - it's in the RNA.

While DNA determines the majority of genetic instructions, RNA is still crucial in a living organism for regulating certain chemical processes. It appears that minor changes in these small strands of RNA might have radically altered the virulence of Y. pestis, turning a mild disease into the ultimate killer. In particular, it appears that it was the so-called sRNA molecules - strands of RNA whose function we still don't entirely understand - that were responsible for the transformation of Y. pestis.

Intriguingly, the researchers discovered 150 specific sRNAs in Y. pestis and Y. pseudotuberculosis that are mostly only found in the Yersinia species. Six of these sRNA molecules were found in Y. pseudotuberculosis but not Y. pestis. The researchers speculate that these six sRNA molecules were lost during the rapid evolution Y. pestis, and it's likely their absence that allowed the bacterium to become the Black Death.

As for how we determined that Y. pestis was indeed the Black Death bacterium, credit for that goes to researchers at the University of Tubingen, who explain their methods in a press release:

The sample was taken from skeletons from a London plague cemetery. The working group in Tübingen, led by Dr. Johannes Krause used a new technique of "molecular fishing" – enriching plague DNA fragments from tooth enamel and sequencing them using the latest technology. In this way, the fragments were connected up into a long genome sequence – which turned out to be identical to modernday plague pathogens...The researchers were also able to show that the plague DNA from the London cemetery was indeed medieval. To do that, they examined damage to the DNA which only occurs in old DNA – therefore excluding the possibility of modern contamination.

Via PNAS and PNAS. Top image is The Triumph of Death by Pieter Bruegel the Elder.